A suborder of hooved mammals (see artiodactyla) comprising the sheep, cattle, goats, deer, and antelopes. They are characterized by a four-chambered stomach (see illustration). The oesophagus empties into the first and most forward chamber, the reticulum. This communicates freely with the second and largest chamber, the rumen. Here food is fermented by the large population of anaerobic bacteria and protists; cellulose and other normally indigestible plant materials are broken down by the microbial enzymes, such as cellulase. Periodically the animal regurgitates material from the rumen and chews it before swallowing it once again; this process, known as ‘chewing the cud’, helps to break down fibrous food. The digesta enters the third chamber, the omasum, from the reticulum. Here water and some nutrients are absorbed, before the contents pass to the fourth and final chamber, the abomasum, which functions rather like a normal stomach, secreting acidic abomasal juice and digestive enzymes from its walls.

Ruminantia: section of the stomach of a ruminant